Simple Introduction to CNC Machining
Mike Vande Weghe
Attached are some handouts that should help you in creating CNC programs to machine your parts. The first one gives a good introduction to milling machines and some basic cut motions.
The second one covers how to select the cutting speeds and feeds so that you achieve a good finish on your parts and so that you don’t break a tool or damage the machine.
When calculating the speed, keep in mind that our machine has a max spindle speed of 5000 RPM (which actually is relatively fast for milling machines), so depending on your tool size it may be necessary to run a little slower than the optimum.
When choosing feed rates, use a value of .005 inches per tooth for aluminum, and .010 inches per tooth for plastic or wax. The third handout shows the coordinate systems on a CNC milling machine, and discusses accuracy. Our machine has an accuracy to 4 decimal places, although if you specify more it will round it off for you.
The final two handouts are from our machine’s user manual, and they describe the instruction codes and also how to verify programs in the software simulation mode.
Here’s the basic procedure for creating programs:
1. Choose the shape and size of stock that you will machine your part from. Determine how you will clamp your stock so that it can be held in a way that won’t interfere with the cutting tools. Pick an origin for your part. other dimensions are referenced from.
This will be the point that all Typically you will set the
origin to be the front left-hand corner of the part, on the top
surface. This is the point that the machine will use to make all its
moves in relation to.
3. Make a detailed drawing of your part, showing the dimensions of
each cutout, and the positions of the cutout relative to the part
origin. You will need to dimension both the lateral offsets (X-Y
positions) and the vertical depth of cuts (Z positions).
4. Choose what size tool you will use for each of the cutouts. If
you need to make a .250″ slot, then the natural choice is a 1/4″ end
mill. If that slot has a rounded bottom, then you will want a 1/4″
ball-end mill. By using a tool size that matches the cut size you
will have to program fewer movements of the tool. Here are some of
the tools we have available:
End Mills: Ball-end Mills: 1/8″
5. Write the G-codes to move each tool to the positions you want.
Each G-code movement will be a position for the tip of the tool to
move to. You will have to calculate where to position the tip in
order to leave behind the edges you want. Don’t cut too deep in a
single pass: you should limit the cuts to be no more than 1/3 as deep
as the diameter of the tool. Deeper slots will require multiple
passes. Group the G-codes by tool, so that you do everything you need
with one tool before changing to the next tool. That will save you
the time of changing tools more than necessary when you run your
program. Don’t forget to raise the tool when you move from one area
of your part to the next area, unless you are making a cut!Keep in mind that G-codes are simply the same sort of movement
instructions as a machinist would perform on a manual milling machine;
you are just telling the milling machine where to move for each step,
referenced to your part’s origin.
I’ve installed the Benchman software on the right-hand PC in HBH 2202,
so you can load your program and verify it using the instructions in
on the milling machine.
Please verify your programs before you bring them to run
Mike Vande Weghe
Institute for Complex Engineered Systems
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